This invention relates to a transmission error correction code appending device which serves as a transmission error countermeasure part for use in a coding device used in carrying out digital-transmission of a motion picture through a variable bit rate line in, for example, a video conference system.
In ITU-T (Telecommunication Standardization Sector of International Telecommunication Union: previously CCITT) and related standardization organizations, a variable bit rate coding system is studied as a video transmission system of a B-ISDN (Broadband Integrated Services Digital Network) age. Inasmuch as use is made, in the B-ISDN, of an ATM (asynchronous transfer mode) transmission in which information is treated as a form of ATM cells, it is possible to realize not only conventional constant bit rate video coding but also variable bit rate video coding. However, transmission errors, such as bit errors and cell losses, inevitably occur in a transmission line. It is therefore necessary on transmission of video information to take transmission error countermeasures in coding and decoding layers and in an ATM adaptation layer (AAL) positioned between each of the coding and the decoding layers and a transmission layer.
In a paragraph 2.5.2.4.1 (Correction method for bit errors and cell losses for unidirectional services) of ITU-T Recommendation (previously CCITT Recommendation) I.363 (B-ISDN ATM ADAPTATION LAYER (ALL) SPECIFICATION), a correction method for the bit errors and the cell losses is disclosed which method uses forward error correction (FEC) and octet interleaving on transmitting the video information as the ATM cells from a transmitting side to a receiving side through the transmission line. That is, a combination of forward error correction codes and an octet interleaver of the ALL type 1 is used as a transmission error countermeasure part of the transmitting side in taking the countermeasure for the transmission errors which occur in the transmission line for transmission of the ATM cells. The octet interleaver is for use in a constant bit rate video transmission and carries out an interleaving operation known in the art.
In Japanese Unexamined Patent Publication No. 63-287222 (namely, 287222/1988) under the title of "Packet Generator", another correction method is disclosed which also uses a combination of error correction codes and an interleaving circuit in the transmitting side in taking the countermeasure for the transmission errors which occur in the transmission line for transmission of packets.
A cross interleaver used in a compact disk device carries out operation which is similar to that of the octet interleaver. The cross interleaver has a capability of correcting a burst read-out error of a head of the compact disk device.
According to the correction method using either the octet interleaver or the interleaving circuit, error resilience can be obtained in case of the constant bit rate video transmission. A maximum delay of either the octet interleaver or the interleaving circuit can be regulated or assured in case of the constant bit rate video transmission. However, the maximum delay of either the octet interleaver or the interleaving circuit can not be regulated or assured in case of a variable bit rate video transmission. This is because supply of data to either the octet interleaver or the interleaving circuit is unavoidably interrupted or stopped during an interrupted time interval in case of the variable bit rate video transmission. The interleaving operation of either the octet interleaver or the interleaving circuit is inevitably interrupted or stopped during the interrupted time interval. The interrupted time interval varies due to a predictive coding state of a coding device which carries out the variable bit rate video coding. The interrupted time interval is, for example, 1/30 seconds (namely, 33 milliseconds) when the coding device carries out a frame drop or subsampling of a digital video signal having 30 frames per second.